The definition of NPSHA is simple:
NPSHA = (Static head + surface pressure head - the vapor pressure of your product) - (The friction losses in the piping, valves and fittings.)
- But to really understand it, you first have to understand a couple of other concepts:
- Cavitation is what net positive suction head (NPSH) is all about, so you need to know a little about cavitation.
- Vapor Pressure is another term we will be using. The product's vapor pressure varies with the fluid's temperature.
- Specific gravity play an important part in all calculations involving liquid. You have to be familiar with the term.
- You have to be able to read a pump curve to learn the N.P.S.H. required for your pump.
- You need to understand how the liquid's velocity affects its pressure or head.
- It is important to understand why we use the term Head instead of Pressure when we make our calculations.
Head loss:
- You will have to be able to calculate the head loss through piping, valves and fittings.
- You must know the difference between gage pressure and absolute pressure.
- Vacuum is often a part of the calculations, so you are going to have to be familiar with the terms we use to describe vacuum.
To convert surface pressure to feet of liquid; use one of the following formulas:
· Inches of mercury x 1.133 / specific gravity = feet of liquid
· Pounds per square inch x 2.31 / specific gravity = feet of liquid
· Millimeters of mercury / (22.4 x specific gravity) = feet of liquid
There are different ways to think about net positive suction head (NPSH) but they all have two terms in common.
· NPSHA (net positive suction head available)
· NPSHR (net positive suction head required)
NPSHR (net positive suction head required) is defined as the NPSH at which the pump total head (first stage head in multi stage pumps) has decreased by three percent (3%) due to low suction head and resultant cavitation within the pump. This number is shown on your pump curve, but it is going to be too low if you are pumping hydrocarbon liquids or hot water.
Cavitation begins as small harmless bubbles before you get any indication of loss of head or capacity. This is called the point of incipient cavitation. Testing has shown that it takes from two to twenty times the NPSHR (net positive suction head required) to fully suppress incipient cavitation, depending on the impeller shape (specific speed number) and operating conditions.
To stop a product from vaporizing or boiling at the low pressure side of the pump the NPSHA (net positive suction head available) must be equal to or greater than the NPSHR (net positive suction head required).
As I mentioned at the beginning, NPSHA is defined as static head + surface pressure head - the vapor pressure of your product - loss in the piping, valves and fittings .
In the following paragraphs you will be using the above formulas to determine if you have a problem with NPSHA. Here is where you locate the numbers to put into the formula:
· Static head. Measure it from the centerline of the pump suction to the top of the liquid level. If the level is below the centerline of the pump it will be a negative or minus number.
· Surface pressure head. Convert the gage absolute pressure to feet of liquid using the formula:
· Pressure = head x specific gravity / 2.31
· Vapor pressure of your product . Look at the vapor pressure chart in the "charts you can use" section in the home page of this web site. You will have to convert the pressure to head. If you use the absolute pressure shown on the left side of the chart, you can use the above formula
· Specific gravity of your product. You can measure it with a hydrometer if no one in your facility has the correct chart or knows the number.
· Loss of pressure in the piping, fittings and valves. Use the three charts in the "charts you can use" section in the home page of this web site
· Find the chart for the proper pipe size, go down to the gpm and read across to the loss through one hundred feet of pipe directly from the last column in the chart. As an example: two inch pipe, 65 gpm = 7.69 feet of loss for each 100 feet of pipe.
· For valves and fittings look up the resistance coefficient numbers (K numbers) for all the valves and fittings, add them together and multiply the total by the V2/2g number shown in the fourth column of the friction loss piping chart. Example: A 2 inch long radius screwed elbow has a K number of 0.4 and a 2 inch globe valve has a K number of 8. Adding them together (8 + 0.4) = 8.4 x 0.6 (for 65 gpm) = 5 feet of loss.
In the following examples we will be looking only at the suction side of the pump. If we were calculating the pump's total head we would look at both the suction and discharge sides.
Let's go through the first example and see if our pump is going to cavitate:
Given:
· Atmospheric pressure = 14.7 psi
· Gage pressure =The tank is at sea level and open to atmospheric pressure.
· Liquid level above pump centerline = 5 feet
· Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.
· Pumping =100 gpm. 68°F. fresh water with a specific gravity of one (1).
· Vapor pressure of 68°F. Water = 0.27 psia from the vapor chart.
· Specific gravity = 1
· NPSHR (net positive suction head required, from the pump curve) = 9 feet
Now for the calculations:
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings
· Static head = 5 feet
· Atmospheric pressure = pressure x 2.31/sg. = 14.7 x 2.31/1 = 34 feet absolute
· Gage pressure = 0
· Vapor pressure of 68°F. water converted to head = pressure x 2.31/sg = 0.27 x 2.31/1 = 0.62 feet
· Looking at the friction charts:
· 100 gpm flowing through 2 inch pipe shows a loss of 17.4 feet for each 100 feet of pipe or 17.4/10 = 1.74 feet of head loss in the piping
· The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet
· Adding these numbers together, 1.74 + 0.6 = a total of 2.34 feet friction loss in the pipe and fitting.
NPSHA (net positive suction head available) = 34 + 5 + 0 - 0.62 - 2.34 = 36.04 feet
The pump required 9 feet of head at 100 gpm. And we have 36.04 feet so we have plenty to spare.
Example number 2 . This time we are going to be pumping from a tank under vacuum.
Given:
· Gage pressure = - 20 inches of vacuum
· Atmospheic pressure = 14.7 psi
· Liquid level above pump centerline = 5 feet
· Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.
· Pumping = 100 gpm. 68°F fresh water with a specific gravity of one (1).
· Vapor pressure of 68°F water = 0.27 psia from the vapor chart.
· NPSHR (net positive suction head required) = 9 feet
Now for the calculations:
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings
· Atmospheric pressure = 14.7 psi x 2.31/sg. =34 feet
· Static head = 5 feet
· Gage pessure pressure = 20 inches of vacuum converted to head
· inches of mercury x 1.133 / specific gravity = feet of liquid
· -20 x 1.133 /1 = -22.7 feet of pressure head absolute
· Vapor pressure of 68°F water = pressure x 2.31/sg. = 0.27 x 2.31/1 = 0.62 feet
· Looking at the friction charts:
· 100 gpm flowing through 2.5 inch pipe shows a loss of 17.4 feet or each 100 feet of pipe
·
· or 17.4/10 = 1.74 feet loss in the piping
· The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet
· Adding these two numbers together: (1.74 + 0.6) = a total of 2.34 feet friction loss in the pipe and fitting.
NPSHA (net positive suction head available) = 34 + 5 - 22.7 - 0.62 - 2.34 = 13.34 feet. This is enough to stop cavitation also.
For the third example we will keep everything the same except that we will be pumping 180° F. hot condensate from the vacuum tank.
The vapor pressure of 180°F condensate is 7 psi according to the chart. We get the specific gravity from another chart and find that it is 0.97 sg. for 180° F. Fresh water.
Putting this into the pressure conversion formula we get:
· pressure x 2.31/sg. = 7 x 2.31 / 0.97 = 16.7 feet absolute
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings
NPSHA (net positive suction head available) = 34 + 5 - 22.7 - 16.7 - 2.34 = -2.74 feet.
We need 9 feet, so the pump is going to cavitate for sure.
If you are given the absolute and vapor pressures in psia, and you forgot how to convet to feet of head; you can use the following formula, providing you know the specific weight of the liquid you are pumping :
· Pp = Absolute pressure expressed in psia. In an open system, Pp equals atmospheric pressure, Pa, expressed in psia.
· Pvpa = Vapor pressure expressed in psia.
· W = Specific weight of liquid at the pumping temperature in pounds per cubic foot.
Subject: The affinity laws for rotary, positive displacement pumps 13-6
The affinity laws accurately predict the affect of changing the speed of a centrifugal or rotary pump, and they also do a fairly good job of predicting the affect of changing the diameter of a centrifugal pump. In another paper (02-01) we discussed the affinity laws as they apply to centrifugal pumps, but in this paper we will look at their use with rotary pumps.
Rotary pump designs include: gear, vane, lobe, progressive cavity, screw, etc. They are more commonly know as positive displacement (PD) pumps and act very different than centrifugal pumps:
· PD pumps do not have a best efficiency point (B.E.P).
· There is no impeller shape (specific speed) to consider.
· There is no system curve to match.
· Their capacity is a constant even if the head changes.
· Unlike a centrifugal pump, if you were going to fill a tank with a PD pump you would fill the tank from the bottom rather than the top to save energy costs.
Take a look at the following two curves. The one on the left describes a centrifugal pump curve with the curve shape determined by the "specific speed" number of the impeller. The curve on the right describes the curve we get with a typical Rotary Pump.
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